An operational amplifier in the form of a semiconductor integrated circuit generally consists of an input stage, a biasing circuit, and an output stage. The input stage is usually a differential amplifier circuit.
It is often desirable that an operational amplifier utilize a very low power supply voltage. This severely constrains the voltage range of the common-mode portion of the differential input signal supplied to the input stage of many prior art bipolar operational amplifiers. For example, consider a conventional operational amplifier in which the input portion of the differential-amplifier input stage consists of a pair of NPN input transistors whose bases receive the differential input signal and whose emitters are coupled together and through a current source to a low voltage supply. The collectors which provide an amplified differential signal are coupled through respective current sources to a high voltage supply. The power supply voltage V.sub.b is the difference between the levels of two voltage supplies. P. Gray et al illustrate such an operational amplifier in "Recent Advances in Monolithic Operational Amplifier Design," IEEE Trans. Circs. and Systs., vol. CAS-21, May 1974, pp. 317-327.
In looking at the acceptable range over which the common-mode portion of the input signal may vary in the foregoing type of operational amplifier, neglect the typically small voltage drops across the current sources. Let V.sub.BE represent the standard forward base-to-emitter voltage of an NPN transistor when it is just turned on. If the common-mode portion is less than about 1 V.sub.BE above the low supply voltage, at least one of the input transistors is non-conductive. The input stage does not provide proper signal amplification. The same occurs if the common-mode portion is slightly above the high supply voltage because at least one of the input transistors is forced into saturation. Consequently, the common-mode input range is limited approximately to V.sub.b -V.sub.BE. The input signal cannot use the full supply voltage range. Since V.sub.BE is about 0.6-0.8 volt, the input signal is restricted to a narrow voltage range in low power supply applications.
Of interest are the complementary differential amplifiers described by K. Yokoyama in U.S. Pat. No. 4,357,578 where the differential input signal is supplied to the bases of a pair of NPN transistors in one input portion of the input stage and to the bases of a pair of PNP transistors in another input portion. While it might superficially appear that using complementary pairs of transistors would extend the range of the input signal from the low supply voltage rail to the high supply voltage rail, this is not necessarily the case. The problem is considerably more complex. The only complete differential amplifier disclosed by Yokoyama does not achieve rail-to-rail capability.